Many different types of printing have been invented, a large number of which are presently in use. Known forms of printers have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.
In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles, has become increasingly popular primarily due to its inexpensive and versatile nature.
Many different techniques of ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore. “Non-Impact Printing: Introduction and Historical Perspective”. Output Hard Copy Devices. Editors R Dubeck and S Sherr, pages 207-220 (1988).
Ink Jet printers themselves come in many different forms. The utilization of a continuous stream of ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.
U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electro-static field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al)
Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.
Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclose ink jet printing techniques rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.
As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high-speed operation, safe and continuous long-term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables.
In the parent application, U.S. Ser. No. 09/112,767, there is disclosed a printing technology that is based on micro-electromechanical systems (MEMS) devices. In particular there is disclosed a printing mechanism that incorporates a MEMS device. There is also disclosed a method of fabricating such a mechanism.
The fabrication of MEMS devices is based on integrated circuit fabrication techniques. Very generally, a sacrificial material is deposited on a wafer substrate. A functional layer is then deposited on the sacrificial material. The functional layer is patterned to form a MEMS component. The sacrificial layer is then removed to free the MEMS component.
Applicant has found that topography of a MEMS chip is very important. The components are required to move. It follows that the topography must be such that sufficient clearance is provided for movement of the components. This means that such features as nozzle chambers must be deep enough to provide for functional movement of an actuator positioned in the nozzle chamber.
There are, however, problems associated with deep topography. This problem is illustrated in Figures A and B of the drawings. In Figure A there is shown a substrate 1 with a layer of sacrificial material 2 positioned on the substrate 1.
One problem is immediately apparent. It is extremely difficult to achieve a uniform deposition on side walls 2 and a floor 3 of the cavity 4. The fluid dynamics of the deposition process is the primary reason for this. As a result, a portion of the sacrificial material within the cavity 4 tends to taper in to the side walls 2.
Accurate etching of the sacrificial material relies on a high image focus on the layer 2. It will be appreciated that this focus could be lost in the cavity 4, due to the depth of the cavity 4. This results in poor etching within the cavity 4.
Etching is carried out using a device that etches in steps. These are usually 1 micron in depth. It follows that each stepping process removes 1 micron of sacrificial material at a time. As can be seen in Figure B, once a required part of the layer 2 has been removed, a part is left behind in the cavity 4. This is called a stringer 5. It will be appreciated that the stringer 5 is difficult to remove and is therefore an undesirable result. The Applicant has conceived the present invention to provide a printhead chip that incorporates MEMS components that are spaced a predetermined distance from a wafer substrate so that sufficient ink ejection can be achieved. The predetermined distance is such that the chip topography avoids the problems described above.